I. Field of the Invention
The present invention is directed generally to a self-contained, disposable cartridge-type electrochemical test cell for use with an associated test instrument having an integral temperature stabilized autogenous electrode calibration and measurement system and, more particularly, to a method and system for controlling and stabilizing the location of the calibration material with respect to the electrode system so that calibration can be accomplished automatically and thereafter displacement of the calibration medium by the sample readily accommodated.
II. Description of the Related Art
The field of diagnostic medicine is fast becoming more sophisticated and complex. The ability to make rapid or immediate diagnostic determinations characteristic of the current condition of a patient so that the proper emergency steps may be taken in a timely manner to improve or stabilize the condition of the patient, for example, during surgery or during the treatment of traumatic injury has become very important. Blood gas determinations including the partial pressures of oxygen (PO.sub.2), carbon dioxide (PCO.sub.2), acidity or alkalinity (pH) and concentration of certain electrolyte species such as potassium (K.sup.+) in the blood are examples of extremely important instantaneous indications of respiratory deficiency, efficiency of inhalation therapy, renal function and other vital bodily processes.
Blood gas determinations heretofore have been made utilizing stationary clinical laboratory instruments that have large reference electrodes and pH, CO.sub.2 and O.sub.2 electrodes. The instruments must be periodically calibrated using a calibration system which is cumbersome in size and intended for use only at a specific temperature. Operation of the instrument is also generally restricted to a specific known temperature, e.g., 37.degree. C. The reference and pH electrodes must be calibrated using a liquid system. The CO.sub.2 and O.sub.2 electrodes can be calibrated using either a liquid medium or a calibration gas. In addition to periodic recalibration of the instrument, control samples also need to be analyzed to ensure continued proper operation; these samples also have restrictive temperature ranges for use. The specific composition of typical liquid control or calibration fluid systems is such that the reference or known equilibrium partial pressures of oxygen and carbon dioxide are temperature dependent and so occur only at the specific storage temperature. Operating or using a liquid-based calibration system at a temperature other than the designed temperature may introduce a decided amount of error into the readings. Gaseous CO.sub.2 and O.sub.2 may also be used to calibrate CO.sub.2 and O.sub.2 sensors in such instruments, but that requires the need for obtaining and for storage of cylinders of compressed gases.
A calibration and measurement system which is small and portable and that makes use of a calibration system that is temperature independent would be very desirable. Reducing the temperature dependence of the calibration system as by temperature stabilization of the amount of contained dissolved or dissociated gaseous species of interest in media for a variety of applications has been demonstrated.
In this regard, it has been found possible to create a packaged system that provides a stable concentration of a gas in a calibration medium despite changes in the temperature of the calibration medium or solvent within a reasonable range of ambient temperature. Such a system is illustrated and described in copending application Ser. No. 07/806,495 of David W. Deetz and Russell L. Morris, filed Dec. 13, 1991 and assigned to the same assignee as the present invention. To the extent necessary for an understanding of the present application, material from that application may be deemed incorporated by reference herein. That temperature-independent system involves the use of an additional separate, reversible equilibrium compensating source containing an amount of the gas or gases of interest packaged along with the calibration medium. The additional source known as a "reservoir" acts in the manner of a buffer to control changes in the partial pressure of the gas or gases of interest in the atmosphere of the package including the atmosphere contacting the calibration liquid. The changes in partial pressure can be tailored to compensate for changes in the solubility of the gas or gases of interest in the calibration medium over a designed range. The system can also be used to control change in the partial pressure of a species of interest as a function of temperature change.
Both the calibration medium and the reservoir are contained in separate gas-permeable enclosures within an outer, common enclosure such that gaseous species may be readily exchanged with respect to the common atmosphere of the sealed outer enclosure but such that the media themselves do not contact each other. The reservoir equilibrium is designed to have a more sensitive temperature dependence and the enclosure of the reservoir to be more permeable to the gas(es) of interest than those of the calibration system and container in order that the reservoir system react more quickly to temperature changes and thus to dominate changes produced in the calibration container. Temperature changes which produce an increase or decrease in the partial pressure of a species of interest in the reservoir medium will cause a corresponding increase or decrease in the partial pressure of that species in the common atmosphere at a rate that will, in turn, anticipate and compensate changes in the calibration system material.
For example, the reservoir medium is designed, upon heating, to expel amounts of a gas or gases into the package atmosphere anticipating the reaction of the sample by raising the partial pressures of these gases in the common atmosphere at a somewhat faster rate than they would be lost from the calibration medium, thereby preempting the thermodynamic driving force for the gases to leave through the permeable shell of the calibration container. Conversely, if the system cools and the solubility of the gases of interest in the calibration medium increases, the reservoir acts to reverse the phenomena of the heating mode and reabsorbs the gas or gases into the reservoir medium from the package atmosphere at a somewhat faster rate than the reabsorption in the calibration medium thereby lowering the partial pressure of the gas or gases of interest in the common atmosphere to eliminate any driving force for the gas or gases of interest in the common atmosphere to permeate the calibration enclosure and dissolve in the calibration medium. This preserves the resulting concentration of each such species of interest in the calibration medium regardless of the direction of temperature change within a designed limited ambient temperature range.
Typically, the calibration medium and/or the reservoir medium are solutions of selective solvents with or without complexing agents or buffers. In the case where CO.sub.2 is the species cf interest, both the reservoir and the sample media may be aqueous solutions of CO.sub.2. A system where the sample pH is 7.4 and the reservoir is buffered to a pH of 8.6, for example, exhibits good temperature/concentration or (pCO.sub.2) stability in the 20.degree. C. to 30.degree. C. range; but (pCO.sub.2) is quite temperature dependent for a reservoir pH above 9.0, or below 8.2.
It being further recognized that while the copending cross-referenced application addresses the physics and chemistry of temperature related calibration media composition stabilization and control, it remains necessary to stabilize and control the locus of the calibration media materials themselves with respect to the sensor electrode system to achieve proper electrode function and automate calibration. In this regard, it is desirable that the calibration medium or media be stored or available over one or more of the electrodes as required for automatic calibration. In addition, the calibration materials must be readily displaceable for the subsequent sample to be subjected to analysis just after calibration so that the sample and calibration media do not interfere with each other.
Accordingly, it is a primary object of the present invention to provide a disposable, self-contained automated calibration and sample testing enclosure that stabilizes the calibration material in contact with selected electrodes until calibration is concluded, yet allows easy displacement by the sample solution to be analyzed.
Another object is to provide a calibration media of a consistency commensurate with maintaining a desired location during storage and through calibration.
A further object of the present invention is the provision of flow control and storage volumes for used displaced calibration fluid.
A still further object contemplates a containment system that maintains calibration material over sensors in a disposable test cartridge used for measuring blood gases and pH, as well as other analyses.
An additional object of the invention is to provide a containment system that maintains calibration fluid over the sensors of a disposable test cartridge by the provision of a flow cell region over the sensors with flanking constrictions sufficient to contain an aqueous or other calibration fluid, yet large enough to allow blood flow.
Yet an additional object contemplates a containment system that maintains calibration material over sensors in a disposable test cartridge used for measuring blood gases and pH using both aqueous and non-aqueous calibration material solvents.
Yet still another object contemplates a containment system that maintains calibration material over sensors in a disposable test cartridge used for measuring blood gases and pH using a gel stabilized dispersion of aqueous and/or non-aqueous calibration materials.
These and other objects will become apparent to those skilled in the art who persevere through this specification in light of the drawings and claims.